1. Field of Invention
This invention relates to a ring illuminator for an optical measuring apparatus.
2. Description of Related Art
Illumination of an object to be measured plays an extremely important role in obtaining a clear image of the object to be measured in an optical microscope. The illumination is important when attempting to optically focus on a portion of the object to be measured. The illumination is also important when capturing an image of the portion using an image processing type measuring apparatus such as a measuring microscope, a tool microscope, a projector or a three-dimensional image measuring apparatus. These devices are particularly useful in inspecting and measuring a shape and a size of the object to be measured based on the captured image of the object to be measured.
Known illumination methods used with such image processing type measuring apparatus or the like include a vertical downward radiation illumination method, which radiates illumination light to the object to be measured from substantially directly above the object to be measured. However, in many cases, the vertical downward radiation illumination method is used for measuring an object to be measured which has a relatively simple shape. Accordingly, when measuring an object to be measured having a complicated shape, such as, for example, a step-like object to be measured having a large number of edge portions, shadows of the edge portions often cannot be clearly detected.
To solve such a drawback, a ring illuminator has been proposed that can clearly detect shadows of edge portions. The ring illuminator operates by radiating illumination light to the object to be measured from a direction that is inclined at a given angle with respect to an optical axis of an optical system of the image processing type measuring apparatus.
A known optical fiber light source is typically used as the light source for the ring illuminator. In general, this optical fiber light source guides illumination light radiated from a halogen lamp or the like through optical fibers. However, the halogen lamp or the like also has disadvantages, including a large power consumption, a short lifetime and/or a slow response rate when controlling the lighting intensity and upon being turned on and off.
Such ring illuminators typically adjust luminance and an illumination angle of illumination light radiated to the object to be measured. One method for accomplishing this forms fiber light sources into groups, and, for each group, independently controls the lighting intensity and activation of that group. To accomplish this independent control, separate lamps are provided for the groups of light sources and the intensity and activation of each lamp must be separately controlled. Alternately, shutter devices or the like, which allow light to be controllably transmitted or interrupted, are located either in the midst of, or at end portions of, the optical fibers. However, the shutter devices for each group must likewise be separately controlled. However, this necessitates a large number of lamps or shutters and the structure becomes complicated. Hence, the illuminator becomes overly large and the manufacturing cost increases.
On the other hand, recently, light emitting elements, such as, for example, light emitting diodes (LED), have been attracting attention in view of the characteristics of these sources, such as, for example, rapid responsivity, a long lifetime and the like. Such light emitting elements have begun to be used as light sources in various technical fields, along with the enhancement of luminance of the light emitting elements. For example, a ring illuminator has been proposed that uses light emitting diodes as light sources to solve the above-outlined drawbacks of the above-mentioned halogen lamps or the like to enable control of the lighting intensity and activation of the light sources.
For example, a ring illuminator is proposed in U.S. Pat. No. 5,690,417 (the 417 patent). In the ring illuminator of the 417 patent, a large number of light emitting diodes are provided as light sources. These light emitting diodes are concentrically arranged in a plurality of circular arrays, such as the five arrays used in an embodiment of the 417 patent. Respective light emitting diodes are mounted such that their light emitting directions are set towards the object to be measured. Further, in the 417 patent, a method is disclosed in which, as a modification of a focusing method, the light emitting directions of respective light emitting diodes are arranged parallel to an optical axis, where Fresnel lenses are arranged downstream of the light emitting directions to focus light on the object to be measured.
Further, in the ring illuminator disclosed in the 417 patent, the light emitting diodes are formed into groups of respective circular arrays and circumferential sectors. The lighting intensity and activation of each group and sector can be separately controlled. Accordingly, it is possible to controllably adjust the lighting intensity and activation of the light emitting diodes arranged in the plurality of arrays. Hence, the distribution of light to the object to be measured can be properly adjusted.
However, in the ring illuminator of the 417 patent, the light beams radiating from respective light emitting diodes that face the object to be measured have intrinsic divergence angles. Thus, those light beams spread out before reaching the object to be measured. Hence, the illumination efficiency is not sufficiently enhanced. Therefore, it is necessary to install an extremely large number of light emitting diodes to ensure the necessary illumination is obtained.
Further, the 417 patent discloses mounting Fresnel lenses in front of light emitting diodes to enhance the illumination efficiency by focusing the illumination light on the object to be measured. However, this does not correct the above-mentioned intrinsic divergence angles of the light emitting diodes. Hence, it is difficult to achieve a remarkable enhancement of illumination efficiency even with this technique.
Further, such Fresnel lenses have intrinsic focal lengths. However, a distance (i.e., the operable distance) between the object to be measured and the ring illuminator changes as the object to be measured changes. Consequently, the focal length and the operable distance of the Fresnel lens are displaced from each other. This results in the illumination light spreading, which lowers the illumination efficiency. Further, to maintain the illumination efficiency, the operable distance cannot be changed. Hence, it is difficult to obtain an optimum illumination light for every object to be measured.
Further, another known ring illuminator controls an illumination angle with respect to an object to be measured and an illumination direction of the illumination light to clearly detect conditions of edges and a surface of the object to be measured. In this known ring illuminator, plural types of rings are prepared. In each type of ring, light emitting diodes are mounted in a ring shape corresponding to specific illumination angles. The light emitting diodes of these rings are simultaneously or separately turned on to control the illumination angle. Further, the light emitting diodes are formed into groups in the circumferential direction for every ring. The lighting intensity and activation of the light emitting diodes of these circumferential groups are controllable so that the distribution of light to the object to be measured can be properly adjusted.
However, in these known ring illuminators, in the same manner as the above-mentioned 417 patent, due to divergence angles of light beams that radiate from respective ones of the light emitting diodes, the illumination efficiency is not sufficiently enhanced. Hence, it is necessary to mount a large number of light emitting diodes to ensure the necessary illumination. Further, it is necessary to mount plural types of rings, where different rings provide different illumination angles. Consequently, the structure of the illuminator becomes complicated.
On the other hand, the types of objects to be inspected or measured by an image processing type measuring apparatus or the like are extremely varied. The objects to be measured include electronic parts, such as printed circuit boards (PCB) or the like, mechanical parts, semiconductor parts, printed matter and the like. Further, the objects to be measured also have various surface colors. In the image processing type measuring apparatus or the like, a shape or the like of the object to be measured in an image captured using a CCD (Charge Coupling Device) or the like is recognized by detecting position of edges or the like within the image. By illuminating the colored object to be measured using illumination light having a hue (i.e., a tone of color) that corresponds to a surface color of the object to be measured, the contrast within the captured image can be improved. The contrast between elements within an obtained image is emphasized so that the detection accuracy of edges in the image can be further enhanced.
In the ring illuminators discussed above that are capable of controlling the hue of such illumination light, a plurality of several different types, such as, for example, three different types of light emitting diodes, have respective different light emitting colors, such as, for example, red (R), green (G) and blue (B). In such ring illuminators, lighting intensity or activation is controlled for the light emitting diodes of each color to control the hue of the illumination light.
However, in the ring illuminators discussed previously, since the illumination efficiency is low, it is necessary to arrange a large number of light emitting diodes. As a result, the ring illuminator becomes overly large and the manufacturing cost increases. Further, as described above, it is possible to control the hue of the illumination light by arranging several types of light emitting diodes having different light emitting colors. In such ring illuminators, the light beams emitted toward the object to be measured from the light emitting diodes of respective light emitting colors are combined on the object to be measured. However, the distances and the angles of the different color light beams of the respective light emitting diodes on the object to be measured are not the same. Hence, the hue of the generated illumination light is not uniform across the object. Consequently, an image of sufficiently high accuracy cannot be obtained.